Embodiments of the invention relate generally to magnetic resonance (MR) imaging and, more particularly, to minimizing phase errors in MR imaging data.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. As appreciated by those skilled in the art, one or more radio-frequency (RF) pulses are generally employed to create the excitation field, B1, which is applied to the substance or tissue, thus manipulating an ensemble of spins thereof.
After application of the B1 excitation field, a signal emitted by the ensemble of spins is acquired and processed to form an image. Depending on the technique employed, the ensemble of spins may be subjected intervening acts prior to acquisition of the image signal. There are a variety of imaging techniques employed in the MR setting.
For example, echo planar imaging (EPI) is a fast imaging technique often used in the field of MR imaging. Generally, during the implementation of an EPI technique, an entire 2D k-space data set is acquired using one or more “shots,” where each shot typically acquires multiple k-space lines by a sequence of readout gradients with alternating polarities. EPI generates “snapshot” images and has been employed with various MR imaging applications, including diffusion weighted imaging and functional MR imaging (fMRI).
EPI techniques, however, can be affected by several drawbacks. For example, due to the readout gradients having alternating polarities, typically every second line of data is traversed backward in the k-space. Accordingly, such data is typically time-reversed before a Fourier transform is applied thereto. The fact that there is almost always asymmetric modulation corrupting the MR signal (due to eddy current, receiver filter asymmetry, concomitant field etc.) and the need to time-reverse every other echo leads to alternating signal modulation between even and odd echoes, which results in the well-known Nyquist ghost artifact. Another example of the drawbacks in EPI is the shifting of an imaged object over several images taken at different time points due to the drifting B0 field.
Hardware improvement and pre-compensation (e.g., gradient pre-emphasis to reduce eddy current) can be employed to reduce the effects of ghost artifacts such as the Nyquist ghost artifact. Nevertheless, one or more ghost correction methods are still required during image reconstruction to further reduce the Nyquist ghost to an acceptable level when an EPI technique is employed. Such correction methods are typically called phase correction methods as they correct or minimize phase modulation or errors along the readout, which is usually the dominant term over magnitude modulation.
One common phase correction method collects non-phase-encoded reference data via a reference scan before collecting imaging data via an imaging scan. Using the non-phase-encoded reference data, a phase difference (i.e., static modulation) between even and odd echoes can be determined. To minimize Nyquist ghosting artifacts, this phase difference is removed from imaging data collected via the subsequent imaging scan. This type of phase correction method is often used in static EPI, where EPI images of a single time point are acquired following the reference scan.
Other correction methods may be used when the EPI technique employed is a dynamic EPI technique, such as fMRI, where a time series of EPI images are collected. Often, dynamic EPI techniques generate additional modulation along readout due to factors such as temperature-related drift, thereby resulting in an increase of ghost level over time (i.e., ghost drift). Typically, the phase correction method discussed above with respect to static EPI cannot account for such additional modulation. To address this ghost drift problem, often a navigator-based correction method is employed. For example, non-phase-encoded navigator echoes can be collected with the EPI data at each temporal frame to calibrate the additional modulation between odd and even echoes. Using navigator echo data, which is elicited by navigator echo pulses, the per-temporal-frame modulation can be corrected during image reconstruction. If a non-phase-encoded reference scan is also employed, static modulation measured from the reference scan can also be corrected during the image reconstruction.
With regard to the navigator echoes, navigator echo pulses are typically incorporated into the scan echo train as its first few echoes (e.g., 3-6 echoes) primarily for signal to noise ratio (SNR) consideration. The navigator-based correction method, however, can have drawbacks. For example, navigator echoes typically prolong the echo train and therefore reduce the maximum number of slices per repetition time (TR). More importantly, it assumes that the additional modulation that the center echoes (corresponding to echoes covering the center of the k-space, which contribute to majority of the signal energy) experience is the same as that predicted by the navigator echoes. When this assumption is not true, the modulation of the center echoes will not be well corrected, thereby still leading to significant drift of the Nyquist ghost.
It would therefore be desirable to provide an apparatus and method to minimize at least Nyquist ghosting in EPI.